1. Field of the Invention
The present invention relates to methods for aligning a substrate in a lithographic apparatus, a method for diagnosing a manufacturing device, computer programs to perform the methods and a data processing device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When a substrate is processed by a lithographic apparatus, an accurate positioning of the substrate is desirable to enable an accurate processing (e.g., applying a pattern onto the substrate) by the lithographic apparatus. Commonly, such an aligning is performed in steps. In a first step a geometric reference of the substrate is detected, which might, e.g., comprise markings at an edge of the substrate, the edge of the substrate itself, some points at the edge of the substrate etc. Starting from these points, an area (or a plurality of areas) are determined in which alignment marks of the substrate are expected. Such area is referred to in this document as a window. The term window is thus in this document to be understood as an area on the substrate in which the alignment mark is expected. Also, the term window may be understood in this document as a capture range of a sensor to detect the alignment mark. In a well-dimensioned system, the capture range of the sensor to detect the alignment mark may at least equal the area or window in which the alignment mark is expected. To avoid any misunderstanding, it is remarked that the window is commonly not marked or outlined on the substrate itself, thus may be considered an imaginary window. Once the area(s) or window(s) has/have been determined, in the window(s), the position of the alignment mark(s) is measured. Thus, first a coarse approximation of the position of the alignment mark is determined, by determining a position of the window, and then in the window a more accurate position measurement is performed.
Thus, when aligning, a stepwise approach is followed: first, using the wafer edge, a pre-alignment is performed. The pre-alignment provides for an area in which the alignment mark may be expected, in other words a position of the window on the substrate. Then, in a second step the position of the alignment mark in the window is detected, which step is commonly considered part of the alignment. Commonly, to be able to accurately align the substrate, a plurality of alignment marks are applied, e.g., four alignment marks for a 1 dimensional measurement, or two alignment marks for a 2 dimensional measurement. To further improve accuracy, positions of additional alignment marks can be taken into consideration.
An implication of this method of alignment is that it is desirable to have a measurement system which meets high specifications to measure the position of the alignment mark in the window. On the one hand, a size of the window is desired to be large, as the expected position of the alignment mark might show fluctuations due to an inaccuracy in a determination of the position of the window, and inaccuracy in the geometric references used for the determination of the window, etc. On the other hand, accuracy requirements on a lithographic apparatus are high and in particular a positioning accuracy or alignment accuracy of the lithographic apparatus is in general high to allow an accurate positioning of a following layer, e.g., on top of a previous layer. The higher the accuracy requirements for the aligning of the substrate, the higher in general an accuracy of the position measurement of the alignment mark is to be. Therefore, requirements on a measurement system to measure the position of the alignment mark in the window, increase: on the one hand, a large size of the window is required to cope with tolerances of a varying origin, while on the other hand a highly accurate position measurement of the reference mark is required.
A further aspect is that in a manufacturing environment where lithographic apparatuses may be used (e.g., a manufacturing of electronic semiconductor circuits), it is not uncommon to make use of lithographic apparatuses and other equipment from a multiplicity of vendors. It is in such an environment not unimaginable that a particular substrate is first processed by a lithographic apparatus, and then (e.g., for applying a following layer) processed by a different lithographic apparatus. Lithographic apparatuses of different types or different manufacturers may make use of different alignment methods. Therefore, a tolerance at the position of the alignment mark may be high: as an example, a substrate may be processed by a first lithographic apparatus, the processing by the first lithographic apparatus, e.g., comprising an application of one or more alignment marks on the substrate. Then, after suitable developing steps, the substrate may be processed by a second lithographic apparatus, the processing comprising an aligning of the substrate to the second lithographic apparatus. As the reference marks have been applied by the first lithographic apparatus, additional tolerances may come into existence due to differences between the lithographic apparatuses due to, e.g., a different method for determining the position of the window on the substrate, different geometric references applied for the determining of the position of the window on the substrate, different accuracies and errors of each of the lithographic apparatus, etc. Due to all these sources of errors, the area in which the alignment mark is to be looked for will increase, thus bringing forward a desire to increase the size of the window.
A still further aspect is that a high throughput of the lithographic apparatus is desired, which translates into a fast detection of a position of the alignment mark, which translates into a simple position measurement of the reference mark in the window. Current measurement techniques, however, have a limited dynamic range, i.e., a limited range between the accuracy, which is to be achieved, and the maximum range over which the measurement can be performed. Thus, given a certain required accuracy, the size of the window cannot be increased above a limit with which the measurement system is able to cope.